How to Write Apartment Numbers in an Address
For U.S. mailing addresses, USPS Publication 28 specifies recognized secondary-unit designators such as APT and gives placement rules for apartment information. This guidance is explicitly U.S.-postal. International address formats can follow different national standards and should not be forced into USPS form.
For U.S. postal addresses, include the appropriate secondary-unit designator and apartment identifier on the delivery-address line in USPS-compatible form.
Key details
Important caveats
Scope Boundary
This guidance is explicitly U.S.-postal. International address formats can follow different national standards and should not be forced into USPS form.
Further guidance
Practical Application
For U.S. mailing addresses, place the apartment identifier on the delivery-address line using a USPS-recognized secondary-unit designator such as APT when applicable. Do not generalize USPS formatting to countries with different national address standards.
Reader Check
Is this a U.S. postal address, and are the secondary-unit designator and apartment identifier placed where USPS expects them?
Recommendation
For U.S. postal addresses, include the appropriate secondary-unit designator and apartment identifier on the delivery-address line in USPS-compatible form.
Worked Example
For a U.S. delivery line, use a USPS-compatible form such as “123 MAIN ST APT 4B” rather than inventing punctuation or moving the apartment number to an unrelated line.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- Publication 28, section 213: Secondary Address Unit Designators (opens in a new tab)United States Postal Service · Official U.S. postal guidance for apartment and secondary unit designators